You're right. My JOBO press kit for C-41 doesn't give a calculation either... But my E-6 kit says to increase developing times by 4% (multiply your dev time by 1.04 exponentially each time) so I would assume its very similar in C-41.
Personally I find it very difficult to measure out the C-41 kits into smaller batches than they are intended, (as in my kit makes a 1 liter bottle of developer and I wouldn't try to separate out smaller batches as its all powder.
The E-61 liquid kits I might, but again once you break the deal, air gets into even the un-mixed concentrate and degrades it.
I've found it's better to just keep exposed rolls in my fridge until I get enough to process out the whole chemistry and do it at once. Otherwise time factors come into play, I ended up destroying an otherwise good roll because I let my E-6 kit sit for a month before using it after mixing.
I just don't shoot enough film for this to make sense, hence why after I kill this JOBO kit, I'm going all B&W ... color chemistry is just too expensive... And doesn't keep long enough for a home user... Or they don't sell it in small bottles for home users, I would gladly spend $100 if they sent me like 10 portions of sealed developer I could then break the seal one at a time and keep the chemistry for a year and only develop 2 rolls at a time and he happy, but they aren't smart enough to do that...
Mamiya: 7 II, RZ67 Pro II / Canon: 1V, AE-1, 5DmkII / Kodak: No 1 Pocket Autographic, No 1A Pocket Autographic | Sent w/ iPhone using Tapatalk